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Title THE SPECTROMETRIC DETERMINATION OF SOME BETA PARTICLE AND CONVERSION ELECTRON ENERGIES

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Author O'Kelley, Grover D.

Publication Date 1951-05-13

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BERKELEY, CALIFORNIA DISCLAIMER

This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. /( UNCLASSiFiED UCRL-1243 MAY 1 5 1951 THE SPECTROMETRIC DETERNINATION OF SOIvlE BETA PARTICLE 3 AND CONVERSION ELECTRON ENERGIES

Grover Davis O'Kelley A. B. (Howard College) 1948

DISSERTATION

Submitted in partial satisfaction of the requirements for the degree of

DOCTOR OF PHILOSOPHY

in Chemistry .':~- in the '." GRADUATE DIVISION of the

UNIVERSITY OF CALIFORNIA

. Approved: ......

• ...... • ...... a ..... 0 ...... " ......

...... • ...... Committee in charge

Deposited in the University Library ...... ! ...... "... Date Librarian TABLE OF CONTENTS

Page

I. INTRODUCTION • • • • 4

'0' II. APPARATUS . • • • • • • • 15

:f, ('r: III. EXPERnmNTAL . • -, .. • • .. • • • • • 40

{ • r~ A. The Decay Scheme of h242 • • • • • 4.3 Ii' ~j B. Radiations of Am241 · • • • • 53 C. Decay SCheIlle of Cm:'42 • • • 57 6 D. Decay Schemeof~22 hOur' Np2.3 and 44 day Np2.34 • 59

E. Radiations of Pu24.3!;l.ndPu238. • • • • 65 203 2OO F. Radiations of Pb and Pb • • • • • • • 70 209 G. Beta Energy of Pb • • • • • • • • 74 H. Neutron Deficient Isotopes of ZircQnium and

their Decay,Products. • • • • • • • • 78 IV. ACKNOWLEGMENT. • • • • • • • • • • 92 v. REFERENCES. , • • • • · • • • • 93

-2- THE SPECTRONETRIC DETERMINATION OF SOHE BETA PARTICLE

AND CONVERSION ELECTRON ENERGIES Grover Davis OtKelley Radiation Laboratory and Department of Chemistry University of California, Berkeley, California .Ij'

ABSTRAC':f

A double . focusing' beta spectrometer having a radius of 25 cm has been constructed. The theoretical transmission for a resolving power of 1 1/3 percent is about 1 percent of 41T. The spectrometer and auxiliary equipment are described. The decay ofAm242 has been investigated, using the beta spectrometer, and a decay scheme is postulated. Schemes are also proposed for the decay of~241, em242, NP236, and Pb203 • The radiations of the heavy nuclides Np234, Pu24.3, Pu238, Pb209, and Pb200 have also been measured. The radiations from neutron deficient isotopes of zirconium and their decay products have been determined, and a decay scheme is postulated for the decay of Zr 8'l.

-3- THE SPECTROMETRIC DETERMINATION OF SOME BETA PARTICLE AND CONVERSION ELECTRON ENERGIES

Grover Davis 0 'Kelley Radiation Laboratory and Department of Chemistry University of California, Berkeley, California

-... '

I. INTRODUCTION

In recent years the spectroscopy of radioactive nuclides has become one of the most fruitful fields of nuclear research. Spectro- metric measurements of energies and intensities are necessary to

test our present concepts of (t, ~, and Y processes, and further develop

mentalong theoretical li~es is hindered by the fact that relatively few investigators have used such a precise approach to the study of radioactive disintegrations. According to the Table of Isotopes of Seaborg and Perlmanl there are more than 700 radioactive nuclides· known, yet Mitchell's review of disintegration schemes2 compiles only about 50 which are reasonably well known in the region below lead. Thus, there is a need for careful spectrometric work, rather than the less reliable absorption methods usually employed. Spectrometric measurements are particularly useful in the heavy region, since these isotopes frequently decay in several different ways, and if some experimentally determined disintegration energies are availa- bIe, others may be calculated through the use of closed decay cycles.

This method has proved very useful in extending the information on electro.n capture decays. The systematics of alpha radioactivity3,4 have been a valuable aid in obtaining unmeasured disintegration energies, and have made it possible to predict the radioactive properties of unknown species. By the use of such methods important advances in the study of heavy nuclei have been made, the most noteworthy being the -5-

rather complete and consistent picture of the energy surface in the

heavy region3,4 which has been constructed. However, the available data on beta disintegration energies '. leave much to be desired, and it is of considerable importance to

measure such schemes with precision comparable to that obtainable in the measurement of alpha particle energies, if unknown energies are to be calculated precisely. The usual beta ray spectrometer is also very useful for mru(ing measurements of gamma energies, as des- , cribed below. Once sufficient data are compiled on t he decay schemes of electron capture isotopes, it will perhaps be possible to make the systematics of electron capture decay of Thompson5 more quantitative, since one would have available the comparative half-lives and electron capture transition energies, if the total decay energy were known

from a decay cycle and the daughter energy levels were known from the decay scheme. It has become customary to call instruments for studying the electron spectra of radioactive isotopes "beta ray spectrometers." Since these devices also yield information about gamma rays by measuring the internally or externally converted electrons, the name "nuclear

spectrometerll has been used by Langer and Cook.6 Before discussing these instruments further, it seems well to describe briefly the theories

of beta decay and gamma emission. Beta diSintegration is considered to be a complex transformation of a neutron into a proton or vice versa, with the simultaneous emission or absorption of an electron and neutrino. The theory was developed in its simplest form qy Fermi,7 who used the Heisenberg concept that the -6- neutron and proton are different states of the same particle. It is postulated that the neutron and proton interact with the combined fields of electron and neutrino in such a way that an electron is created and a neutrino radiated when a neutron changes to a proton. Conversely, a positron and neutrino are radiated ,men a proton changes to a neutron. The probability for the emission of a beta particle with momentum between T) and T) + dT) is given by Fermi as p(Tj)d'f) = G2/2TT.3IMI2F(Z,T)T)2(eo _ e}2d'rl, where T) is the momentum of the electron in units of lIloc, e is the total energy in units of IIloC2, and eo is the maximum energy of the distribution. The quantity (eo - e) represents the momentum and energy of the neutrino. The factor G is given by G = (g/moc2) (moc/h).3, where the constant g measures the strength of the interaction between the nucleons. and the electron.neutrino field, and is analogous to e in the quantum electrodynamic treatment of the interaction of electrons with the electromagn7tic field (light). This "Fermi constant;" g, must be determined empirically. The factor IMI2 represents the square of the matrix element con taining the wave functions of the initial and final states of the neu tron and proton and the form of the interaction. For allowed transitions thiS factor is of the order of unity, but is smaller for forbidden' tr?TI sitions, and may sometimes be dependent upon the energy of electron and neutrino. This behavior arises from the expansion of the matrix ele ment into a series of terms, which become successively smaller if they -7-

do not vanish; only the first nonvanishing term is important to the

order of the transition. If the first term does not vanish it is close

to unity, regardless of the interaction type and energy. Such transi- I '. tions are allowed. For certain transitions the initial and final nuclear wave functions are such that the first term in the M-expansion is zero. In such cases the second term assumes importance, and the,transition is first-forbidden. Since successive terms in the expansion decrease by the

factor iiR, where 'Yiis the average momentum and R is the nuclear radius in units of h/mc, the square of the matrix element for first forbidden transitions is smaller by 10-2• Of the five relativistically invariant forms of the interaction, the tensor and axial vector forms seem to best fit the experimental data. Such interactions lead to the Gamow-Teller selection rules:8

l1I = 0, + 1, no ° -> 0; no parity change (allowed) l1I 0, + 1, ± 2; parity change (1st forbidden) LXI = 0, ± 2, ± 3; no parity change (2nd forbidden)

Wigner9 has modified the selection rule by dividing the allowed transitions into "favored',1 and tfunfavored" transitions. The favored transitions are thought to be those in which both parent and daughter

nuclei have the same spacial wave-functions. The unfavored transitions

then become characterized by a lower matrix element due to a change . in the spacial wave functions with respect to symmetry or configura-

tion. In terms of ft values discussed below, the favored transitions would have ft "-3000,' vlhile the unfavored ones v10uld have ft "vl05• From empirical data alone, the classifications of allowed (unfavored) and -8-

first forbidden transitions thus overlap. The nuclear shell model of

Feenberg and HammacklO is sometimes helpful, since parities are obtainable from the shell theory. ,> The decay constant for a transition is obtained by integrating

the momentum probability distribution from zero to ~ax' or

T) max 2 3 }. • 1n2/tl/2 • J .. P(~)d~ • G /2n 1M12 f(Z,eo) o

Since f(Z,e o) is dependent only on the charge and maximum energy, it can be calculated for any iso-

top~ for which these quantities are known. The product of f and the half-life in seconds, usually denoted ft, isa measure of the magnitude of a2/2n3IMI2, and enables one to classify beta transitions as to their orders of forbiddenness. Values of ft fall into groups in which each

degree of forbiddenness increases ft by a factor of 100. A very good

rev:'.9Vl of beta decay theory and compilation of ft values is given by KonOPinSki,ll while more up-to-date tables of ft values have been com pi3:ed by Feenberg and Hammack,lO and Nordheim.12 The calculation of 13 f(Z,eo) has been treated recently by Feenberg and Trigg. Experimental comparison of a momentum distribution with the theory }..I is best done with the aid of a Fermi-Kurie plot,14 which also yields

the maximum energy. If the experirnental distribution N( T)) is expressed as particles per unit time for a constant momentum interval; then -9-

For a given type o~ interaction the factor G/2n3 will be constant; the matrix element will also be constant over the distribution.except for special forbidden transitions whose matrix elements are energy de- . pendent. These cases are rare, and it will be seen that the Fermi- Kurie plot serves to detect such situations. Therefore, we may write the following equation for a beta spectrum having an allowed shape: N (11) '" (const) F(Z,T}),f (eo _ e)2 The Fermi-Kurie plot of N( 'l)/.,fF(Z,'I) 1/2 as a function of e is a straight line with an intercept at e". eo. The units of N, T}, and e do not influence the maximum kinetic ( lergy one obtains from eo, so that instead of T'i, one might use H, HP, or magnet current of a beta ray spectrometer, and the plot could /ery well be a function of E in Mev, yielding the maximum kinetic energy Eo directly. However, relativistic units are usually quite convenient~ Use of the Fermi-Kurie plot requires precise knowledge of the form of the function F(Z,'l). This coulomb correction factor, which is the Dirac wave function for an unbound charged particle in a coulomb field, was given by Fermi7 in his original article on beta decay. Omitting momentum-independent factors, which do not affect the shape of the Fermi-Kurie plot, this function is:

F(Z,'l) = (const.)~ enY Ir11 + S + iy)12, where S "" (1 ,,; y2)1/2 - 1 y "" r Za '" Z/137 for f5 emission ~Za '" -Z/137 for ~ emission y =({1 + T}2)1/2/~ "" (y) (e /T}) There are no complete tables of the complex gamma fUnction, and so a direct evaluation must be bw means of series. To avoid the tedium of -10- such computations, various approximations have been made. The most reliable approximation is due to Bethe and Bacher:15

F(Z, Tj) = (const.). 2 Tf( f(l + Tj2)(1 + 4 y2) _~ S -2Tl'Y L' ::J 1 - e This approximation is good to about 3 percent below atomic number 80, and is still the best approximation at atomic number 90, although the error is about 5 percent. However, it is the variation in the percen- tage error with momentum, and not a constant error which would distort the shape of a Fermi-Kurie plot. Feister16 reports ~ program of computation and tabulation of the exact function f(Z,11) = 1\2F(Z,n) under way at the National Bureau of Standards. It is planned to compute f(Z,1\) for both negatrons and positrons for all values of ~from 0.050 to 7.00 (0.67 to 3100 Kev) at sufficiently close intervals so that intermediate values of Tj may be directly interpolated. Feister's report includes some of the preliminary results at intervals of 10 Z,up to Z= 90. Although the Fermi theory is. derived on the basis of a simple beta transition, the Fermi-Kurie plot may be resolved into components, if several components are present in the decay. If two components are present, the plot of the high energy one will be a straight line beyond the upper limit of the softer component, but the Fermi-Kurie plot of the mixture will rise above the straight line at energies below the maximum energy of the soft component. The straight line obtained by extrapolating the hard component to lower energies is subtracted from the curve' of the mixture, recalling that the Fermi-Kurie plot of the soft component is ¥CNtotal - No)/r(F against, where No is -11- obtained from the extrapolated high energy component. However, such

·a procedure should be used with caution, since scattering from the source and spectrometer l.m.llS can cause an excess of electrons at low

energies \{hich may resemble a separate component. If two beta particles decay from the same level in the parent to different levels in the daughter nucleus, the presence of gamma rays will indicate the validity of the resolution into components. End points of lower energy

components can generally be obtained with reasonable precision in this way, except Where the low energy beta particles are of such low

intensity that they must compete with backscattered electrons from the more intense components. It is good practice, therefore, to use very thin samples and backings.

Theoretically, the Fermi-Kurie plot for infinite and finite re-

solving pOvTer will coincide except in the region of the endpoint. In practice, the resolving power is generally dependent upon source

thickness, and it is possible for a thick source and poor resolving power to distort a straight-line Fermi~Kurie plot, or to straighten

out a curved plot. This is particularly important in cases where small. deviations from linearity are significant, as in studies of forbidden·

spectral shapes. Owen and PrJJllakoff,. 17 and vlu and Feldman18 have

studied the effect of poor resolution and thick sources, which in

some cases turns out to be quite pronounced. When a nucleus is left in an excited level as a result of radio-

active decay, gamma ray bombardment, or other excitation process, the probability for gamma ray emission to a lower level becomes high. The

quantum theory of radiation employs the classical concept of a -12-

radiating source as an oscillating electric or magnetic multipole, and employs instead of the classical frequency w, the quantityhw

times the probability that a gamma ray shall be emitted. Since the radiation carries away angular momentum, application

of conservation of momentum to the system of nucleus and photon leads one to establish the following selection rule:

where --I and It are the angular momentum vectors in units of~ for the states between which an electric or magnetic 2~pole transition occurs.

The radiation of lowest order (i.~., the most probable) between two levels with angular momenta I and I' will be an electric or magnetic mul tipole of order 2~ in which ' ~ = tI - I'I Note that I and If are the scalar quantities.

'\fuen one also considers parity, i.~., what happens to a given eigenfunction if the sign of all the particle coordinates is changed, the following complete selection rules result:

Order =A 1 2 3 4 5 Electric radiation Dipole Quadrupole Oct. l6-pole 32-pole .£ 1 2 3 4 5

~I Parity change yes no yes no yes

Magnetic Radiation Dipole Quad. Oct • l6-pole ..e 1 2 3 4 '. Parity change no .yes no yes -13-

The table also includes values of the order of the transition, denoted by.A. To the table must be added the selection rule that

o ---> 0 transitions are forbidden for all types of radiation. This is a result of the equation for minimum Jl, given above. The present state of knowledge of nuclear structure is not suffic ient to allow calculation of absolute transition probabilities, but various authors have calculated nuclear lifetimes assuming different nuclear models. Segre and Helmholz19 have obtained an approximate formula for the decay constant by simplifying a treatment of the problem due to Bethe:20

loglO Ay =20.30 - 2 loglO (1·3 ••• 2£- 1) ... (2'£ + 1)(1.30 - loglOE) - 2£(0.84 - 1/3 loglOA, where 'Ay is in sec-I, E is the gamma ray energy in Mev, and .£ is the

multipole order for electric transitions. However, withing the accura~ of such a calculation, the decay constant for 2t pole magnetic radia~ tionhas the same probability as electric z'+l pole radiation, the

decay constant of magnetic 2i. pole is obtained from the formula by

setting A. =1+ 1. A nucleus may also decay to a lower energy state by giving the energy difference between the two states to an electron in the

K, L, M ---shell of the aton.. Thus, monoenerget~c electrons are

ejected with kinetic energies Ey - EK, Ey -EL, etc., where Ey is the

energy difference between the levels, and EK, EL' ~. the binding

ener~y of the electrons in their respective shells. Such a process is

called "internal conversion." It is important to realize that this is '. a primary process, for internal conversion competes with gamma ray -14-

emission, and the decay constant for the excited state is the sum of the decay constants for the two processes. Thus, A ::: Ay + A e

and TI/2 .. O.693/(Ay + A e) == O.693fo. y(l + a),

where Ay is the decay constant for ga.tilma. ray emission, Ae is the decay I constant for the internal conversion process, and a is the internal conversion coefficient, defined as the number of internal conversion electrons per unconverted . gamma ray_ Axel and Dancoff21 show that when this correction is applied to an equation £or Ay having about , the same energy dependence as Segre and Helmholz's equation, very good agreement exists between theory and experiment. Although the calculation of A relies upon knowledge of nuclear matrix elements and nuclear structure, the' calculation of internal .. conversion coefficient.s depends only upon knowledge of electrodynamics and atomic structure. Internal conversion offers a means of. exploring the multipole order of transitions, since the degree of conversion depends greatly upon the electric and magnetic field in the vicinity of the nucleus where the K-and L-electrons are located. The field decreases with a power of r which is related to the multipole order. Although the calculation of numerical values is a·formidable task, the

theoretical evaluation is well known and has been treated by several workers. Hulm.i2 has treated internal conversion in the K- and L shells due to the field of an electric dipole, using Dirac's 'relati vistic equations. Goertzel and Rose23 have generalized Hulme's treatment to include radiation from multipoles of arbitrary order. An .• extensive numerical calculation of K-shell conversion coefficients far -15-

electric and magnetic multipole radiation up to order 25 has been carried out by Rose and co-workers. 24 Their results have been distri

buted privately in the form of tables. The analytic expressions of Goetzel and Rose23 were simplified

from a computational point of view by Gellman and co-workers,25 w.ho have published preliminary results for the Lr shell only, for the

el~ctric dipole, magnetic dipole, and electric quadropole cases. These preliminary results are exact except that screening has been

omitted. However, a rough screening correction maybe made19 by

replacing Z by (Z-4) when interpolating the curves of Gellman, et ale As a result ofa spectrometric investigation of a radionuclide, the atomic number, gamma ray energy, and ratio of K electrons to L

electrons can be determined. If the transition is isomeric, the half~ life of the gamma transition can be directly meastlred. Even if the

conversion coefficients cannot be directly measured the half-life ctn d

NK/NL ratio are very helpful in assigning a multipole order to the transition. Unfortunately, sufficient calculation of conversion

coefficients have not been made for the L shell to allo~use of the

aK/aL ratio reliably above an atomic number of about 35. Some additional aspects of beta decay and gamma ray emission will

be discussed in connection with the experimental work described below.

II • APPARATUS Before describing the beta spectrometer (or I1nuclear spectro meter") used in the investigations below, it woUld be well to recall

'. some general features of magnetic beta spectrometers. -16-

If an electron in an evacuated space moves in a plane perpendicular to a magnetic field H, it will describe a circular path in that plane.

The magnetic force e(ll~) on each electron is then balanced by the centrifugal force rrri2-/P. Therefore one obtains the following relation, vmich holds relativistically: Hev = m~ P On rearrangement,

p "" mv "" eHP In these equations H is the field in gauss, e is the charge on the electron in emu, P is the radius of curvature in cm, and m is the mass of the electron at velocity v. Since charged particles, such as electrons, often move at right angles' to uniform magnetic fields, HP values are convenient primary measurements. The "magnetic rigidity" or HP is proportional to the relativistic momentum, whence the momentum in units moc becomes • HP = Hp(gauss -em} 1'704.1

After eliminating velocities in the relativistic equations for

momentum and energy, one can relate HP in gauss -em and E in Mev as follows: HP ."" 3336 [E2 + 1.022 E) 1/2 E ::: [0.,2609 + 8.987 x 10-8(HP)~ 1/2 - 0.5108

''!' The inverse resolution R of a beta spectrometer is defined 'as f::::.p/p; the "momentum bandwidth"f::::.p is a measure of the momentum spread of electrons which can strike the detector when the spectrometer is '. adjusted to focus electrons of momentum p. For a monoenergetic electron -17-

line, R is the halfwidth of the measured line divided by the position

of the peak of the line. Thus th~inverse resolution is dimensionless; it is independent of momentum for all magnetic spectrometers. Since the inverse resolution is a constant of the apparatus, and the momentum is proportional to the magnetic rigidity, the momentum •. bandwidth is proportional to momentum, magnetic rigidity, or to any other measurable quantity that is proportional to momentum:

6p '= Rp = (Re)Hp

In the analysis of continuous beta spectra it is therefore necessary to divide the counting rate at each momentum setting b.Y some quantity proportional to momentum, so that the activities are '. normalized to a constant momentum interval. Beta spectrometers may be conveniently divided into two groups,

the flat spectrometers ahd the helical spectrometers. An example of the former is the semicircular spectrometer proposed by Danysz,26

'lfhich is still widely used. Figure 1 shows such a device, 'tmich makes use of a uniform magnetic field perpendicular to the electron beam. The electrons described circular paths, those of equal energy having equal radii. Two electrons diverging from the source S with almost the same initial direction will converge at one and tile same point after traversing approximately 1800 of arc (point A, Figure 1). The central rays Ivill have the same radius of curvature as the diverging rays, but

will strike the image plane at point B of Figure 1. Thus; even for a point source, the reiolving power is limited. Electrons leaving the source at angles to the focusing plane of Figure 1 will have reduced .' -18-

- _-1a- t- -- a s --, p ---

MU 173f

Fig. 1

Electron trajectories in the semicircular beta spectrometer -19-

momentum components in that plane, and their paths projected on the

image plane will be circles of shorter radii than the rays shown in

Figure 1; this effect will increase the low energy "tail" of a mono energetic electron line. It will be seen that for this type of beta spectrometer, the conditions that would improve the resolution (narrow

.• beam aperture, source, and exit slit) all serve to decrease the number of electrons reaching the detector, and will reduce the transmission of the instrument. In all beta spectrometers, a compromise is neces sary between resolving power and transmission. The low transmission of the Danyszspectrometer due to its one dimensional focusing has led to the use of the various helical spectro meters, which utilize the focusing property of an axially symmetric field for electrons emerging from a source located on the axis. The path is a cylindrical helix with the axis parallel to the direction of the field. Such a system forms a convergent magnetic lens. The use of a magnetic lens in beta spectroscopy was proposed by Kapitza;27 and first accomplished by Tricker.28 The high transmission of the two dimensional focusing helical spectrometers is generally counterbalanced by a poorer resolving power than the semicircular instrument. However, recent modifications of helical spectrometers have improved somewhat the relation between transmission and resolving power. It was shown by Svartholm and Siegbahn that it is possible to combine the advantage of space focusing (high transmission) with that of using central rays (high resolving power). Such a spectrometer may be considered a flat spectrometer derived from the semicircular instru ment by replacing the uniform magnetic field by a non-uniform one, in -20-

such ,a "m.y that there is meial focusing as strong as the radial focus-

lng.. S·leg bah n, Sval' tl10 1m .; and >hG ell'. co-ltlOr..:eers1 29-~1..; h ave s t u d·le, d the

theory of this particluar focusing device in considerable detail, .. while Shull and Dennison32 have shovID such focusing to a~ise from

a general treatment of magnetic spectrometers. E.N. l'-~cHillan appears

to have first proposed this IIdouble-focusingll spectrometer although

his results 1'!ere not published at that time.

Because of its good transmission and high resolving pOlfer, such

a double-focusing beta spectrometer has been constructed in this

laboratory, and it is the purpose of this paper to describe this

instrument and some of the measurements for \imich it has been used.

Before proeeeding, a brief reViel,l of the theory of this spectrometer

,ull be given.

Consider an inhomogeneous magnetic field of cylindrical synmietry,

1..li th a plane of symmetry perpendicular to the axis. According to the

theory of the focusing properties of such a field, there is an axial

focusing action ,,;hich decreases outHard from the center of symmetry,

as well as a radial focusing action if the,field does not decrease

faster than the reciprocal of the radius from the axis. If the axial

component of the field in the median plane at a distance I' from the

axis is denoted by H(r), and its derivative vdth respect to I' by

'. H'(r), then an electron leaving a point on the circle I' ::: 1'0 in the median plane and at a small angle to the circle, Hill strike the

cylindrical surface I' ::: ro after an angle ¢r, the radial focusing

angle, and the symmetry plane after an angle ¢z, the axial focusing

angle.. These focusing angles are given by: 29 -21-

which yields the relation

•'.

The radial and axial focusing will be of equal strength if ~r = ~z,

which gives I-1'(ro) = - 1/2 rQH(ro). Thus, we have an optical axis of radius ro, on which the object and image are separated by an angle tr 'fi radians. The requ:' red field varies approximately as, l/fT in the vicinity of the optical axis ro.It is desirable to define the field shape more closely to focus rays diverging from the optical axis at considera-

ble angles ; thus a series expansion' of the axial field component in , the median plane H(r) is employed:

H(r) = HoU + a: ,T- To + ~ - r)20 + • ••] , L- TO t TO where Ho is the axial field component at ro, r is the radius in the

vicinity of ro, and ~ and ~ are constants. From the above expression

relating the derivative of the field to the field at i' = ro,'O: must

be equal to - 1/2, while the other coefficients are at our disposal. Svartholm,30 HcMillan (private communication to Svartholin and Siegbahn), and Shull and Dennison32 have all sho-wn that the coefficient P is the factor which determines the aberration and hence the resolu- tionof such a spectrometer. Values of P from 1/8 to 3/8 have been employed;3l ,33,34 the choice of ~ involves practical considerations -22- of magnet design. Persico and Geoffrion35 propose the use of ~ = 3/16, for which the ratio of transmission topolepiece area is very high.

Figure 2 shows a general layout of a typical double focusing spectrometer. The source and detector are located n(:2 or about 2550 apart, and the entrance slit A is midway on the electron trajectories. The divergence angles are determined by this slit; the .radial beam width is 2'fZ roYl and the axial.beam width is given by 2'(2roY2. Svartholm and Siegbahn29 ,30 have shown the dispersion of the double focusing spectrometer to be twice that of a semicircular one, and an aberration width about half as large. Also, one obtains the interesting result that ·for ~ = 1/8, the radial defocusing is elimina ted, and for ~ = 3/8, the same is true for axial defocusing. When '(1 and '(2 are about the same, it is impossible to reduce the aberration width by a choice of ~between 1/8 and J/8, but one can improve the resolution through the use of an extended aperture. For example, if p = 1/8 (as in the instrument below) one shoulduse a large radial acceptance angle and a relatively smaller axial angle which can be varied to obtain various line widths. In such a case, the transmission varies linearly with the axial aperture, while the line width decreases according to a square la1.f. Two alternative magnet designs are shown in Figure 3. Design (a) allows the use of a large radial extension of the beam, and hence is very suitable fora double-focusing beta spectrometer with P = 1/8. Also, with this choice of field shape, the polepieces may be placed fairly close together, and fringing effects do not become serious. This magnet is eminently suited to theoretical design, and may be -23-

(0)

, /

(b)

s o A

MU 1732

Fig. 2 Double focusing spectrometer: projection of electron paths (a) in the median plane, and (b) in the axial plane -24-

IRON

(0)

(b)

IRON

MU 1733

Fig. 3 Alternative magnet designs for the double focusing beta spectrometer -25- machined to a calculated pole shape with confidence that the magnetio field will have the proper distribution. For these reasons the spectrometer described herein was constructed similar to the closed magnetic circuit spectrometer of Figure 3-(a), even though this design requires more copper , and power for the same radius than the instrument shown in Figure 3-(b). The "double mushroom" magnet of Figure )-(b) is due to Cockcroft36 who used such an arrangement for a large alpha spectrometer. It has the adv.antage of yielding large radii with a magnet of modest physical size, but has the disadvantage of a small radial area, because of the magnet coil. Thus, to obtain the opti mum transmission a large axial aperture must be used, with the result that a ~ of 3/8 is required. In this case the po1epieces are spaced much farther than in the former instrument, and fringing effects become'difficult to cope with except by empirical means. , The po1epieces of our spectrometer were carefully designed theoretically, and machined to an average tolerance of better than 0.1 Percent in the bending zone. A diagram in cross section for half of one po1epiece is shown in Figure 4, together with its theoretical dimensions. Because of the cylindrical symmetry of the device, the other half of the po1epiece cross section is the same. The equilibrium radius chosen was 25.00 cm, or 9.842 inches. An assembly drawing, Figure 5, shows the method of mounting the magnet coil, which consists of 725 turns of number 8 Formex insulated wire, having a resistance of 3095 ohms. The spool upon which the magnet coil is wound is separated from the vacuum tank of the spectrometer by a brass wall, rendered vacuum-tight by rubber gaskets. A small, radar type blower circulates -26- .

.618 REF.

Ro • 9.842 INCHES

Fig. 4 Cross section of double focusing spectrometer polepiece -27-

air around the magnet coil when the magnet power supply is operated.

The temperature of the magnet coil may be estimated by a thermocouple

placed inside the winding, but no heating has been observed, even when

operating at 10 amperes through the coil. Such a current corresponds

to about 18,000 gauss -cm or 5 Mev, and is the limit of the magnet

power supply. As shown in Figure 5, a vacuum tight, brass well is introduced

into the spectrometer chamber for making field measurements in the

median plane.' If it is desired to measure the field in the region on each side of the equilibrium radius, the 'spectrometer vacuum tank

can be let down to atmospheric pressure, and the fluxmeter well may

be released and rotated to each side of its normally fixed position.

An external pointer is adjusted so the position of the search coil

within the well is known precisely. Figure 6 shows the result of such

a field shape measurement, and demonstrates the close agreement between

the actual field and the theoretical shape. The deviations from the

calculated curve at low radii are probably not significant, as the

probe position vms relatively uncertain there. Deviations below a radius

of 8 inches or above 12 inches would only affect the transmission,

leaving the resolving power essentially unchanged.

The power supply for the magnet coil is composed of a bridge type

selenium rectifier, a low ripple, choke input filter, and a suitable

regulating circuit. The operation of the control and stabilizing cir

cuit is as follows. A saturable reactor bank, connected in series with • the isolation transformer supplying the selenium rectifier, can be con

trolled by two pairs of 304 - TL vacuum tubes, so that the voltage -28-

Fig. 5

Assembly drawing of the beta spectrometer A. Airlock for introducing sample probe

B. Airlock gate valve C. Airlock pumpout valve D. Rotating coil fluxmeter motor E. Counter tube pumpout F. Housing for counter high voltage terminal G. Sample holder ring H. Magnet coil and spool J. Brass vacuum wall

Po Diffusion pump manifold -29-

5 10 o - ___-LI~--:- INCHES

fIIU 1'13:5 1.2 2 l! 1; _ I (R ~ R0 ) I· (R - Ro ) . M - 1- I@--.· ... - t Ie ",Q . Rg . Ro ! ..

1.1 H Ho lNetiES

1.0

O. 97.':":.O:--~~"7 .. ~8.':'t·o~" ':"':'--=--~---='--~--~-a~~-~~-"~R:~=I=NO=~='-~-i~o~""'-o""".· ""'-"""---~-I""'I.-0'--;'---1...I2.o

. ". .:.. -~ .Au 1736

Comparison of experimental and calculated field profile -31- applied to the rectifier is varied over a wide range. Two different control voltages are applied to the 304 - TL control tubes. The first is a variable bias, derived from a very stable 600 volt DC power supply; since the bias determines the plate current of the control tubes, the output current to the magnet coil can be adjusted in this way. A second control voltage is applied from a resistor in series with the magnet, which is sensitive to changes in magnet current which may occur, and transfers such changes to the control tubes as a correction signal. This system has the advantage of continuous current variation over the entire range of about 0 - 10 amperes with a single control. A simplified schematic diagram of 'the electronic circuit is shown in Figure 7. In normal operation, the cont;r:-ol bias is obtained through the flRemote Controll! panel, which effectively divides the full current range in~o a series of 10 overlapping intervals and one coars~ range of 0 - 10 amperes. The potentiometer on this unit can be driven by a motor of variable speed, and an auxiliary recording device indicates the magnet current as a function of time on a Leeds and Northrup multi~ pIe input "Speedomaxfl recorder which also plots the counting rate of the detector as a function of time. An exploratory sweep of a, spectrum can thus be obtained quite conveniently, and this device has proved very useful.

For investigating low energy spectra, it is convenient to expand the low current ranges. This is achieved by switching a fixed resistor of approximately 0.1 the resistance of the magnet and its circuits across the output of the power supply, so while the power supply undergoes -32-

r-~----, : CDITROl I I PAUL I L _J IIIV.le. lEt

---l I I I I -, r RfGUUTOR filii: - I I. I .21 21 I I 201 I '1 I I I 81 I I I I I I I I ...... ---~---,J I I I I. I L ______---1 r------, I~ COITROl I 11·19.~A I PAm I I -- ---

'------i: _ "'lET CUAIUT I L-_~',,-n r - -, IfCOIDIIG : :CnTm PAm L_ -.J

L-======t::;-ilt---;:-:-=:;;r RaIIleOIPOTEI1IOIfTER lIP( B

LI ______

., 1"7

,~\

Fig. 7

Simplified schematic diagram of'the magnet power supply -33- an excursion of 0 - 10 amperes in ten steps, the magnet current is about 0 - 1 ampere. The current through the magnet coil is measured by observing on a null-type potentiometer the voltage drop across a 0.1000 ohm,

15 ampere Leeds and Northrup standard resistor. Within the spectrometer are four polystyrene baffles, placed 15,

18, 21, and 237 degrees from the source. The baffles are fixed in aperture to allow a transmission of 1 percent of 41Tfor an inverse resolution of 1 1/3 percent. However, if the source has a large area, this high transmission canno"t be attained; usually transmissions of 0.2 - 0.5 percent can be expected. "With the slit apertures reduced, inverse resolutions of 0.36 percent have been used.

The pumping syst~ consists of a Distillation Products Corporation VMF - 100 oil diffusion pump, backed by a Welch Model 1505-A Duoseal mechanical pump. The airlock through which samples are introduced is exhausted by a small, auxiliary mechanical pump_ The diffusion pump oil is protected by safety devices which turn off the heating element if the cooling water fails- or if the pressure in the vacuum tank exceeds about 30 microns of mercury pressure, as indicated on a National Research

Corporation vacuum thermocouple. With the pumping system described, a pressure of about 3 x 10-6 rom of mercury is obtained under operating conditions. Low Pressure measure ments are made with a Western Electric D-795l0 ion gauge, and a standard Radiation Laboratory ion gauge power supply described elsewhere~37 Detection is accomplished with a Geiger counter of the end window type, which is positioned at the focus of the spectrometer by an -34-

adjustable brass support. This counter tube has a 1/4-inch brass

collimator before the window, which is either a thin 0.0005-inch Nylon sheet or a Formvar foil 30-50 gm/cm2 thick, supported on a Lectromesh grid. The windows are sealed with gaskets of dental dam . rubber.. Since such thin windows require that the tube be filled in

situ, the counter tube is connected to an external evacuating and filling apparatus. Because of the solubility of the plastic windows in alcohol, ethylene is used as the quenching gas, the usual fill being

8 0 8 cm of a mixture of 10 percent ethylene and 90 percent argon. To compensate for the loss of counter filling mixture by

diffusion through the very thin windows a pressure regulating device has been installed, which operates as follows. When filled, the counter tube is connected to a 5-liter reservoir of filling gas at the operat

ing pressure 6f 8~8 cm. Also attached to the counter tube is a mercury vacuum manometer with a YOlfram contact just above the meniscus of the mercury column on the high pressure side. Should the pressure decrease, an electrical connection is made between the wolfram wire and the mercury column, which actuates a solenoid valve through a sensitive relay. This solenoid operated valve allows filling gas to be introduced from a high pressure reservoir until the counter system is again at 8.8 cm. A schematic layout of this regulator is shown in Figure 8 •. The Geiger tube is connected to a quenching preamplifier of low deadtime, the output of which is fed to a standard scaler. In addition to the scaler, a counting rate meter of both linear and logarithmic

response is used for making exploratory sweeps with the mot9r drive -35-

DIFFUSION PUMP E MANIFOLD

MU 1738

Fig. ,8

Schematic diagram of counter tube pressure stabilizer A. Sylphon type needle valves B. Solenoid valve C. 5-liter high pressure reservoir D. 5-liter reservoir at 8.8 cm pressure E. Counter tube pump out valve F. Electric circuit for actuating solenoid G. Counter tube H. Vacuum manometer, with electrical contacts which close with a decrease in pressure -36- unit of the magnet supply. No departure from linearity between the magnet coil current and

H has been observed either from direct field measurements or by measuring the current required to focus various monoenergetic conversion lines. However, there is some residual field which cannot be entirely eliminated, so the calibration equation has the form

HP =:: KI + c, where K and c are evaluated by measuring the magnet current required to focus certain standard electron lines. The residual field contri- bution c is kept constant by passing zero to 1.5 amperes of alternat- ing current through the magnet coil in a demagnetizing cycle, after which the direct current is always increased to avoid hysteresis effects. Figures 9 and 10 show curves of relative field as a function of magnet current, and HP of some standard electron lines against magnet current, respectively. Although the spectrometer was designed for a transmission of 1 percent for a resolution of 1 1/3 percent, the usual transmission is much lower, due to sample preparation, windovl absorption, and possibly other factors. The transmission at low energies is especially poor, since the supporting grid for the thin Formvar lvindow is only

50 to 60 percent transparent. Above 200 Kev, where the Oo0005-inch Nylon windows can be used, the transmission is about 0.5 percent for a resolving power of 1.5 percent. However, in cases where a low transmission can be tolerated, very good resolving power can be obtained by decreasing the axial slit width and using carefully prepared sources.

Some results for various Cs137 samples are shovm in Figure 11. -37-

15.0

10.0

MV 6

5.0

6.0 10.0 CURRENT, AMPERES MU 17a9

, 1"'",

Fig. 9

Fluxmeter reading as a function of magnet current •,

HI' 1000

°0~------~1----~---2~------~3------~4 MAGNET CURRENT, AMPERES

MU 1740

t':

Fig. 10 Typical beta spectrometer calibration curve 10 K line of 0.08013 Mev gamma ray of I131 2. L line of 0.08013 Mev gamma ray of I131 3. K line of 0.2841 Mev gamma· ray of I131 4. K line of 0.3642 Mev gamma ray of I131 5. K line of 0.6614 Mev gamma ray of Cs137 6. K photo1ine of 1/172 Mev gamma ray of Co60 (uranium radiator) . 7. K photo1ine of 1.332 Mev gamma ray ofeo60 (uranium radiator) . -39-

l!. .I 0.9% 0.4%

1.2 %

MAGNET CURRENT

MU 1741

Fig. 11 The K internal conversion line of the Cs137 gamma ray, for three different samples and inverse resolution values. -40-

Photographs of the magnet rack and the electronic equipment are

shown in Figures 12 and 13.

III. EXPERIMENTAL

The beta spectrometer described above has been in r~utine use

for about a year, and has been employed in a variety of investiga- _ tions. The purpose of this section is to describe the author's research with this instrument. Samples are generally mounted on thin Tygon plastic films

20-30~gm/cm2 thick which are coated with a thin layer of gold

or aluminum evaporated in vacuum. Tygon is preferred over other plastics for such applications because of its acid resistance, as is the gold coating. The purpose of the metallic film is to ground

the source to the source holder frame, which in turn is grounded to the spectrometer proper. If this grounding of the sample is"not ob-

served, serious shifts in low energy electron lines will result, particularly in samples which are extremely alpha radioactive.

Heavy alpha emitters consitute a great health hazard in the

quantities employed for the investigations below. Indeed, the hand-

ling of large samples of transuranium elements requires considerably more care than the usual beta or gamma active samples encountered in

beta spectroscopy, and contamination of the surroundings must be studiously avoided. In order to confine the alpha contamination in a small area, a glove box of transparent plastic was constructed "\ by the Health Chemistry group of this laboratory, and is used for mounting the radioactive samples on the source probe. The probe is -41-

...

IN 42

Fig. 12

Photograph of the beta spectro~eter magnet rack Iflr __ t=t ------" I! -- -- t - t

r: Q:" ·...

Fig" 13

\ Photograph of the auxiliary and control equipment for the beta spectrometer -43- introduced into the box through a Wilson seal, and after mounting the sample it can be inserted in the spectrometer airlock with a minimum possibility of contamination. ~ photograph of the glove box with a source in place is sho\~ in Figure 14.

A. The Decay Scheme of Am242 Neutron irradiation·of 475 year Am24l results in the production of a pair of. isomers38,39 Aril242m with a 16 hour half-life known to decay by beta particle emission, and a long lived (100 year) ground state, Am242, which is also a beta emitter and has weak alpha branch ing.. Since strong sources of h242m and Am242 can be prepared by pile neutron irradiation of Am24l , it was possible to examine the radiations of the two isomers of Am242 , particularly Am242m, with x-ray and beta spectrometers. As a result of these investigations, it became possible to characterize not only the beta transitions, but also the isomeric transition and electron capture decay •

.The electron capture has been shown independently to result in the long lived nuclide Pu242 by mass spectrographic examination of the plutonium fraction from a sample of americium which had been subjected to long neutron irradiation.40

A curved crystal x-ray spectrometer proved very useful in analy zing the Lx-ray mixtUJ;'e. This instrument and some of the other measurements for which it has been employed, have been described elsewhere by Barton, Robinson, and Perlman.4l The x-ray spectrometer sample consisted of 2 mg of Am24l which had been irradiated with pile neutrons and repurified chE?IDically. The pure americium was then -44-

N Z ~

. Fig~ 14

Photograph of source probe glove box, showing source and probe in place -45- precipitated as AmF3 and dried to a powder which was sealed in a thin walled, narrow glass capillary. This source gave very sharp peaks which allowed good energy measurements in the spectrometer. The spectrum was scanned with a motor sweep of constant speed, and the impulses from the scaler were recorded on a Streeter-Amet Trafi

counter 0 Approximately 6 hours were required to scan the Lx-ray region; the decay of the lines was followed by scanning continuous ly for about six days. Figure 15 shows the decay over the 6 day period.

The L series x-rays of curium, americium, plutonium, and neptun ium were positively identified from the close agreement with the values calculated by Barton using the Moseley relation. The validity of this extrapolation of x-ray energies into the transuranium region has been established by the work of Barton41 on the L x-rays of plu tonium and neptunium. The curium, americium, and plutonium x-rays were observed to decay with a half-life of about 16 hours, while the neptunium x-rays did not decay appreciably over the 6 day period. Presumably the origin of the x-rays is as follows: the curium x-rays arise from the internal conversion of a gamma ray following the beta transition, those of americium are from the internal conversion of the isomeric transition gamma ray, and the plutonium x-rays most probably arise from a combination of L electron capture and internal conversion processes. The long lived neptunium x-rays are to be ex pected from the internal conversion of the gamma ray of Np237 follow ing the alpha decay of the target material, Am24l (see Section B). -46-

1110 HRS 1715 HRS. I JULY "~OO -= ___ 'I""w I 0-t ( I I 30.0 310 33.0 34.0 35.0

300 Cm L/3, "1 ,

200 PUL/3, j

CmL/32 1853 HRS 28 JUNE I NpLJ3, PuL/3 100 j j 2 NpL/32 PuL/3 I I 6

BACKGROUND

o 30.0 31.0 32.0 33.0 34.0 35.0 SPECTROMETER SCALE READING

MU 1142

Fig. 15

L x-rays from decay of Am242m, showing decay of lines -47-

The relative intensities of the x-rays of curium, americium, and plutonium are of great importance to the formulation of a decay scheme. The observed intensities were corrected for absorption in the source capillary, quartz diffracting crystal, and counter idndow by assum

ing these effects to be equivalent to 150 mg/crrl- of aluminum, and using Allen's compilation of absorption coefficients.42 The xenon

·filled proportional counter efficiency for the various x-ray~nergies

was estimated from the work of Crane and Ghior~o,43 while the reflec tivity of the quartz crystal was assumed to vary as 1/Ji2'~ as calcu lated by Lind, West, and Dumond.44 Barton, Robinson, ana Perlman have found that the relative intensities of the x-rays involving the

LIU level (LQ';l' L~2' etc .. ) agree well idth those of electron bom barded Uranium.4l The intensities of the x-rays from the decay of

• Aro242m were normalized,trucing the relative intensity of the curium Lal line as 100. Here also, there is agreement betwee; intensities

from excitation of uranium by electron bombardment and excitation by the internal conversion process and electron capture in the cases

of the L~l and L132 lines. However; it is also interesting to note that the x-rays involving the LrI and Lrrr levels are in approximately , equal abundance for the plutonium and curium x-rays, but the americium

LIrrx-rays from the isom~ric transition are about five times as abundant as the Lrr level radiation. Selection rules are probably .' in force such that the Lrrr level is especially favored, and may well

.+ be related to the long life of the isomeric state. This unusually

~ large number of LIII transitidhs is noticeable in the conversion electron spectrum, and is in disagreement .'lith Kinsey's assumption45 that the LI level is ahrays the most strongly excited in the internal

conversion process, although in the cases of curium and plutonium x-rays the ratio of LII x-rays to LIII x-rays is about that reported '. by Kinsey. Table l,lists the principal lines observed, their measured •'. energies and intensities.

Table 1 Principal x-ray lines ·in decay of Am242m

X-ray Relative line desig- Level Measured intensity Calculated nation transition energy (corrected)a energy

Pu Letl LIII - Mv 14 •.33 :!: 0.01 68 14.31 L~2 LIII - NIV 17.32 :!: 0 .. 02 22 17.29 LPI LI1 - Mrv 18.34 :!:0.02 54 18.;30 LYI L1I - NIV. 21.52 :!: 0.05 18 21.43

.Am La]. 14.66 :!: 0 0 02 39 140-66 L132 b L~l 18.90 :!: 0.07 8 18.84 LYI b Cm Letl 15.01 :!:. 0.01 100 15,,00 Lf3:2 18.13 :!: 0.04 36 18.08 L~l 1948 :!: 0.02 107 19.38 , LYI 22.78 :!: 0 0 07 33 22.68

aEstimated corrections for crystal and counter window absorption and reflection intensity variation with energy. bNot resolvable from other lines.

Microgram quantities of purified, neutron irradiated americium were used for preparing the beta spectrometer sources. Conducting

Tygon films were used as backings. In the course of the investigation, six different samples were observed and four pile irradiations of Am24l were made. -49-

The beta spectrum of Am242m was determined on each of the four

bombardments and Fermi-Kurie plots of the data were made o Calibra

tion in this energy region was made against the Kelectron .line of

the Cs137 gamma ray, whose magnetic rigidity of 3381 gauss-cm has been

precisely compare9: ,dth the K line of the 04112 Mev gamma ray af'. Au198 by Langero46 Figure 16 shows a typical Fermi-Kurie ploto Table 2 lists the values of the beta energy obtained, from which the final value of 628 Kev is obtained, and the absolute uncertainty is estimated to be ± 5 Kevo

Table 2 Determinations of Am242m beta energy

Experiment Beta energy, Kev

" 1 628 2 628

3 625 4 630

There is some evidence that the beta spectrum of Am242m is co~

plex, with a second component at about 280 Kevo At the present writing, evidence for a 350 Kev gamma ray is not definitive, due to the possi bility of a small amount of fission product activity which may have followed the chemical separations. Attempts to precisely measure the beta energy of the 100 year Arrl-42 isomer failed because of insufficient

activity, using the above samples. However, Dr .. S. Go Thompson kindly -50-

•.

KEV MU 1743

Fig. 16 Fermi-Kurie plot of Am242m beta spectrum -51-

prepared a sample of Am24l,242 with fairly high specific activity of Am242 • From this source, a value of 593 Kev for the beta energy of the long lived isomer was obtained. Lead and copper absorption curves of the electromagnetic radiation • '. from the 16 hour isomer showed only a soft gamma ray of aBout 50 Kev .. and L x-rays, but no detectable hard gamma radiation or K x-rays • A portion of the low energy electron spectrum in which lines were seen is shown in Figure 17. The resolving power of the spectrometer was adjusted to about 0.5 percent, but at the low energies en- count.ered in these experiments, backscattering from the sample backing introduces a smearing effect which is very difficult to eliminate.

The electron lines have not been corrected for window absorption in

Figure 17, but estimates of relative abundances.were made, using win

dow absorption corrections published by Heller, Strucker, and Weber.47 The electron lines are assigned to the L internal conversion of

gamma rays of 53 Kev, 35 Kev, and. 38 Kev, assuming these gamma rays to result from transitions in cUrium, americium, and plutonium, respectively. These assignments are based on the energy differences of

I the lines, which seem to best match the LI, LII, and tIll differences of curium, americium, and plutonium. Such assignments are not con- elusive, since the 'experimental uncertainties are of about the same order as the differences. However, further evidence is obtained from the decay of the lines •. It is observed that the electron lines of the

35 Kev gamma ray decay with a half-life of 15 to 20 hours, while the

lines of the. 53 and 38 Kev gamma rays decay initially with a short half-life of this:order, but gradually tail into a half-life Which is -52-

.'.

2000

Am Lm + Pu LII •

N I

400~------~~------~~------~~~----~------~150 200 250 300 350 ·400 I, MA

MU 174~

Fig. 17

o Low energy electron lines of Am242m -53-

very long. It seems reasonable that the 53 Kev gamma ray should be

associated with the beta transition, since. this is the predominant

decay process, and the 53 Kev gan~a ray is seen in absorption measure-

" .. ments o The 53 Kev gamma ray is approximately 40 percent converted, if it follows the beta decay. This is to be compared ivi th a value

of 50 percent, based on absorption measurement by Crane.48

On the assumption that the 53 Kev gamma ray is 40 percent con-

verted, that the other two g~ rays are 20 percent converted, and that

the L Auger coefficient is about 0.5, the relative probabilities for

the various decay processes can be estimated. A,m242m appears to decay

15 percent by electron capture,15 percent by isomeric transition, and

about 70 percent by beta emission.

Since the 38 and 53 Kev gamma rays were observed to be present in both 16 hour Am2!1.2m and 80. year Am242 , it is assumed that the beta decay of the long lived isomer excites the same 53 Kev level in cm242 as the 16 hour activity, and that the electron capture involves a

transition to the level in plutonium 38 Kev above the ground state. Evidence with the one sample of pure 80 year Am242 investigated thus

far indicates that such is the case, although there is some preliminary

evidence for a harder g~a ray also following the electron capture.

The experimental results discussed here can be summarized as a

decay scheme, "Thich is shol-ln in Figure 18 0

B 0 Radiations of Arn24l

Since 475 year p.m24l is produced in milligram quantities in

nuclear reactors, its chemical properties have been extensively -54-

PU2!B 6.18 em 242

11.38 . 10.65 colc. 234 4.74 P Np 2!,8 5.45 l~m2421 (a) ° .. est. est:· . 0.20 J0.29 colc. . J0.78 colc, Th1 23.4 4.25 U 238.4.96 Pu 242

0.82 0.781 MEV MEV 0.78 MEV 0.646 (b)

o PU 242

( MUI14S

Fig. 18

Proposed decay scheme or Am242 · -55- studied in this laboratory, using weighable quantities of material. Because of its availability and possible use as a counting standard, this nuclide \~s investigated on the beta ray spectrometer. Seaborg and co-workers38 had found that the nuclide Am24l emits gamma rays of about 62 Kev, and in an intensity of about 0.4 per alpha particle. These workers also found x-rays of about 20 Kev, which were the L x-rays of neptunium. Barton, Robinson, and Perlman4l measured these x-rays on the x-ray spectrometer and showed that they indeed arose from transitions in neptunium. (See also Section A above.) The samples of Am24l were mounted as previously described, using thin backings, but macro amounts of material. Both Auger electrons from L type transitions and internal conversion electrons were observed. The L, M, and N conversion lines of a 59.4 ± 1.0 Kev gamma ray are shown in Figure 19. This gamma ray energy checks the absorption value quite closely. The internal conversion coefficient of this gamma ray can be estimated from the electron line intensities. Obtaining the transmission of the spectrometer as described in Section C below, the ratio of L electrons to total alpha particles was found to be 0.28 •. Since the abundances of the two alpha particle groups are not kn01.Jll accurately, it was necessary to calculate the internal conversion coeff;J.cient from the ratio of gamma rays to alpha particles, which ~s taken as 0.4. Thus it appears that this gamma ray is about 40 percent L converted, i.~., aL ~ 0.7. One may also estimate from these data that about 70 percent of the Am24l alpha transitions excite a level in Np237 59 Kev above the ground state, and that the remaining .30 percent of -56-

10 r------..;...~------

COUNTS MIN

O~------~------~----~--~300 350 400 450 l,MA

MU 1746

Fig. 19 Electron spectrum of Am241 -57-

the alpha transitions are to the ground state of Np237• These estimates of abundances are probably accurate to ± 20 percent. The postulated decay scheme is included in Figure 19.

C. Decay Scheme cf Cm242

The nuclide Cm242, an alpha particle emitter of 163 days half- '".... life, can be prepared in microgram quantities by neutron irradiation

of Am24l , and the subsequent decay of Aro242m as described in Section A,

above. Absorption experiments indicate L x-rays and a gamma ray of

about 50 Kev. No hard electromagnetic radiation is detectable.

Intense Cm242 sources were obtained from 1-1. 1/. T. Crane and run on the beta spectrometer, with the result that Auger electrons

and internal conversion electrons were observed (Figure 20). The

internal conversion lines can be assigned to the L, M and perhaps the N conversions of a 43 Kev gamma ray. From measurements of the line intensities, it is possible to estimate the conversion coefficient. With this in mind, the geometry and detector efficiency of the spectre- meter were measured by extrapolating a Fermi-Kurie plot of a sample of Pro147 to low energies, and correcting the low energy points up to the straight line~ The spectrum of Pro147 is known to follow the allowed shape down to about g Kev,49 so such a procedure is valid of scattering effects are not too severe. The sample of Pro147 was about the same size /' and weight as the cm242 source, in order that the "luminosities" would be the same. A further datum required is the relative abundances of

the two alpha groups. Asaro and Perlman 50 have .found the high energy

alpha group to be in 70 percent abundance, while the other, 'which -58-

" . <

AUGER •

pu 238

.' 10

o L-__-L ______~ ______~~ ______~~~---- 200 250 I, MA 300 350

'. MU 1747

Fig. 20 Electron spectrum and decay scheme of em242 -59-

precedes the 43 Kev gamma ray, is present in 30 p'ercent abundance. The

conversion coefficient is calculated as follows:

" (Total electrons detected) conversion coefficient "" 0.3 (Transmission)(total alpha disin- '." tegrations)

For theL conversion electrons ~ this was measured as about

45 tlO percent. The ratio of L conversion to M conversion is about 5.

These data are consistent with the decay scheme shown on Figure 20.

,A consideration of the closed energy cycles relating to the dec~

of Np236 indicated (Figure 21-a) that electron capture was energeti-

cally possible to the extent of 1.21 Mev in addition to the betaemis

sion already known to occur. 51 ,An investigation of this isotope was

undertaken in the hope that both the beta dec~ and electron capture . . branching could be detec~ed and the relative intensities measured.

The samples of Np236 were prepared by bombardment of U235 wi. th

14 Mev deuterons from the 60 inch cyclotron at the Crocker Radiation

Laboratory .. Smaller amounts of Np234 were also produced in these

bombardments. The target material was dissolved and a radiochemically

pure neptunium fraction was separated bY,D.oA~ ,Orth. The final step

of the chemical procedure was an elution of the neptunium fraction from

an anion exchange column with concentrated; HOI.. ,Thus it became possible

to prepare carrier free samples of neptunium of high specific activity.

Beta spectrometer sources were mounted on thin Tygon plastic films about

20-30 gmfcn?- thick. Tygon was very useful. in this application, since

a drop of concentrated HOI may be evaporated on such a film without

mishap 0 • As a precaution against SOUTee eharging, a thin layer of gold -60-

'. 236 l~:5. 5.85 Pu "' 0.51

4.86 Ac22~ Pa 232 ..5.01 (0) . est. 1 est. 0.05 j0.76 calc.. 1.21 calc. Ro 22" 4.05 Th 23~ 4.56 U 236

u 12:'_~-4....;;6:....:..:'2=-.;6=-- Pu :~;2 calc. calc . . 214 4 a 230 5 40 ( b) 81 - 3 est. -- pal "as;. 1.00 1.31 . 1 calc. Pb21~ ____ Th230~ 4.88 4a ..

MU 1148

.. " Fig. 21 , Closed decay cycles involving Np236 and Np234 was evaporated over the plastic .film and the source holder ring, which

was then grounded to the spectrometer frame. When samples of Np236 were examined on the beta spectrometer, Auger electrons, internal conversion electrons, and beta particles

were seen. Figure 22~A shows the electron spectrum obtained. The

-.< LAugerelectrons are far too .intense to be accounted for by the internal conversion of the gamma rays, shown. Fermi-Kurie,plots

(Figure 22-B) of the continuous beta spectrum are comp1ex~ and two

beta groups may be resolved, having energies of 0.51 Mev (59 percent) and 0.36 Mev (41 percent). A pair of internal conversiOn lines corres-

ponding to the K and,L conversion of a 0.150 Mev gamma ray was also' observed.4ssuming this gamma. transition to follow the 0.36 Mev beta transition, the gamma ray is about 100 percent converted, with a

ratio of K electrons to L electrons of about 22. AI. powerful argument in favor of this scheme is the calculated disintegration energy of 0.51 ,MeV' from the closed energy cycles of Figure 21. It was briefly mentioned tha:t the Auger electrons observed were

far in excess of those expected from K and ,L electron vacancies due

to the internal conversion of the gamma rays, or from the electron capture in the Np234 which was also present. These electrons, when, corrected for the presence of Np234 , are a measure of the electron capture in Np236 " However, window corrections for the beta spectro-

meter detector are uncertain in this low energy region, and so it was deemed desirable to use absorption methods to e'Valuate the relative number of electron capture and beta transitions. The radiations were

resolved by beryllium and silver absorption into electrons, L x~rays, -62-

'. L- AUGER A

_N_ K 10001 E.," 177 KEV 5 (Np 234) I(rAUGER LE., .. 150 KEV 1t ~ ( Np 236,

(AMPERES) 1.0 B

10 JI~F '. 5

.' 1.9 2.0 K C K E.,= 803 KEV I E.,.,442KEV (Np2~ t(Np 234, 200 400 40 N N NIl II (elM) 100 200 L 20 I ,M(?) 2.0 2.2 2.4 3.5 3.7 1 (AMPERES) MU 1146 l',

Fig. 22 A. Electron spectrum of Np236 and Np234

B. Fermi~Kurie plot of Np236 beta spectrum C. Additional electron lines from Np234 -63-

and K x-rays, as described elsewhere.52 The relative intensities were corrected for the counting efficiency of the detector and the L AUger coefficient, which was assumed to be 005 from the work of Kinsey.,45 " An estimate of the K electron capture was made, taking into account the internal conversion of the 0015 Mev gamma ray in the Kshell • .. Further, the total.L vacancies calculated from the absorption data minus the number of L vacancies resulting from the K vacancies is taken as the number of L electron capture vacancies. Relating these intensities to the number of beta particles allows the completion of the decay scheme, in which the relative abundances should be accurate to± 20 percent (Figure 23-a)o AI though there is a large amount of L electrcm capture fqr this disintegration energy of 1021 Mev, ather heavy nuclides such as

u23l, Np23 5, and .&m242m have demonstrated predominant capture af L electronso53 ,54,55

Besides the conversion line of the 00177.Mev gamma ray of 404 day

Np234 shown in Figure 22-A, another group of rather prominent lines are found beyond the Np236 beta spectrum which correspond to the K, L; and possibly the M :i,.nternal conversion of a 0 .. 803 Mev gann;na. ray.. These lines decay with a 4 day half-life, and thus are associated with the decay of Np234" The closed decay cycle of Figure 2l-b shows the elec tron capture energy of Np234 to be 1 .. 87 Mev. There is also a line of

low intensity at even higher energy, which has a longer half-life ·than II Np236, and is most probably the K internal conversion peak of a 1.42 Mev gamma. ray.. If a mixture of. Np234 and Np236 is allowed to decay for several half-lives of the latter, it becomes possible to resolve out

L , -64-

40 {J2 0;36 MEV I .

N 200 ELECTRON ., (N 100% CONVERTED) (0) CAPTURE .. 0.15 MEV 1.21 MEV TOTAL ELECTRON CAPTURE TOTAL BETA EMISSION .. 2 , ~ L ELECTRON CAPTURE = 2 ---.,~_ . K ELECTRON CAPTURE

I.83MEV

(b)

0.803

1.42

0.442

0.177 o 234 .U MU.1749

Fig. 23 Decay scheme of (a) Np236 and (b) Np234 the K electron line of a 0 0 442 Mev gamma .ray of Np234.. The electron

spectrum of these gamma. rays is shown in Figure 22-c. Although an

extensive study of the decay scheme of Np234 has not been made, it

seems apparent that this electron capture nuclide has associated with

its decay gamma rays of (5).177 Mev, 00442 Mev, 0 0 803 Mev, and 1 .. 42 Me'i7'o

A rather straight forward interpretation of these data is that the

first three gamma rays are in cascade, with the 1.,42 Mev gammaray

a crossover transition~ This scheme is shown in Figure 23-b.

It would be very interesting to examine the gamma ray spectrmn

of Np234 more thoroughly, and especially to obtain the relative

gamma rayintensitiese

Eo . Radiations of Pu243. and Pu238

The elect~n capture inAJn242m has been show.n40 to result in the long lived plutonium isotope, Pu242, while the beta decay ofAm242m 2 forms cm24 0 If the plutonium fraction is separated from a sample of Am24l which has been subjected to long neutron irradiation, both

Pu242 and. Pu238 are present in about equal amounts by weight,40 the latter plutonium isotope resulting from aJ.pha decay of cm242 .. Alpha

pulse analysis and mass spectrographic measurements of these plutonium

samples showed the alpha particle energy of Pu242 to be 4 .. 88 .Mev and

the alpha particle half-life to be approximately 5 x 105 years, in

good agreement with the alpha decay systematics),4 Energy surface c~nsiderations in this region indicate beta stability for both Pu242 2 8 and Pu238 0 Recently the half-life of Pu 3 has been measured as 57 .' 89.59 ± 0 0 37 years,56 and the alpha particle energy as 5047 Mevo -66-

Neutron irradiation of Pu242 has been shown to result in the 5 .. 0 ±Oo2 hour beta activity Pu243.58,59 The cross section for the

fdrmation of Pu243 by the reaction Pu242(n~Y)Pu243 with slow neutrons

" 59 .,. has been determined as approximately 100 barns • A mixture of Pu242 and Pu238 was irradiated for a short time in the Hanford nuclear reactor, and the plutonium fraction was subs8-

quently repurified by Ko Street, .fro and Do A. Orth. This sample,

now containing Pu243, was mounted on a thin Tygon film and inves-

tigated in the beta spectrometer. Unfortunately~the specific activity of the source was very low; the spectrometer detector indicated only about 110 counts/minute above background at the peak of the beta distribution. Although fairly long counts were made, the statistics were limited to 10-15 percent because of the 5 hour half-life. A.

Fermi-Kurie plot of the smoothed data is shown in Figure 24, in which the beta energy is found to be 0.41 ± 0.02 Mev. Ghiorso finds t'WO gamn.tS. rays of approximately 001 Mev energy, 60 froin pulse analysi,s of the electromagnetic radiation using a xenon filled proportional counter. Because of the low activity of the Pu243 source, a thorough search for conversion electrons could not be made manually on the beta spectrometer, so the motor driven sweep was employed. Two rather questionable peaks were seen in the region of

L electrons from gamm~ rays of about 001 Mev; if these lines are L electron lines the gamma ray energies are 0 .. 095 and 0.12 Mev. Making the assumption that the two gamma rays are in cascade, one obtains a disintegration energy of about 0.,6 Mev for Pu243, from which

the alpha particle energy of Pu243 can be calcUlated by closing the -67-

6~~---r------~-----r------~-----r----~

4 I . ~)!l2 ( If

:,'" 3

0.4ItO.O.2 MEV

0.20 0.25 0.30 0.35 0.40 MEV

Fig. 24 Fermi-Kurie plot of Pu243 beta- speotrUm -68-

decay cycle:

243 P!,U n-> Aml243 a 4.7(calc.) a 5.30

U239 ~ Np239 -> 1.21

However, it is possible that the beta spectrum is comple~ and that the beta decay energy of Pu243 is actually 0.5 Mev; this would lead to

an alpha par~icle energy of 4.6 Mev for Pu243, which is not as con-

~istent with the alpha systematics as the 4.7 Mev value. After the decay of the 5 hour Pu243 activity, some low energy

electron lines remained which had not decayed appreciably. These lines are attributed to Pu238, since the nuclides Pu242 andPu239 also present in the source are too long lived to have been detected in this experiment.

The electron spectrum without cotmterwindow correction is shown

in Figure 25. ~ table of electron energies and their possible interpretation is tabulated below:

E, Kev Interpretation Ny, Kev

23 L;ra + Auger 44.8

24.2 !.rIa 45.2

" 25.9 (1) LIb ('2) 47.7

27.9 LIlIa 45.1 .31.1 (1) LllIb (1) 48.3 40.9 Mlrla 45.2 45.9 Mvb (Na + Oa 1) 48.6 -69-

-"-N I 2

o~--~------~------~------~--~~---200 250 300 350 I, MA

MU 17S1

Fig. 25 Electron spectrum of Pu238 -70-

The .L and M internal conversion lines are definitely assigned, but the

lines indicated as questionable are difficult to intrepret. However,.

one may be fairly certain of the existence of two gamma rays in Pu238,

with energies of 45.1± 0.5 Kev, and about 48 Kev. No attempt was made to measure l':"elative abundances of the various lines. I.t must be re-

called that in the fi~e. no window abso1'p~ion corr.ections were applied,

so the low energy liJ:les are act~y in much higher abundance ~

F\ Radiations .of pb203 a.ndPb200

A study of ~2 hour }>b203 provides an opportunity to look for a

transition in the T1203 d,aughtercommon to the transitionobserved

when T1203 is exeited·· by the beta . decay of .Hg203 • $l:'tis and Siegbahn6l

have carefully investigated Hg203, and find K and L internal conversion

lines of a single. 279.Kev gamma ray with a ratio of K to L electrons of about 3.. ~utz,Pool, and Kurbatov62 found electron lines of a

270 gamma ray in the decay of p~03, as well as a line ~ich they

assigned to Compton electrons from a 470 Kev gamma ray. Itj thus seemed.

worthwhile to investigate the decay of l>.0203mor~ thoroughly in the .. , , '. ", ~

hope that either this di~erepancy could be resolved or established.

The first samp.le i?vestigate~ waS prepared by the . (d,2n) and (d,4n) reactions on thallium, using 60 Mev deuterons from the 184 inch

cyclotron. The chemically separated lead fraction ~s prepared by I! ,D,.q. Karrak~r, and con~ained onl¥ about 50 p,gms of lead as carrier. , The Geiger cquntertube used in the experiments had a mica window

about 3 mg/ cm2 thick, so rather severe window absorption was encountered at some of the very low energies. A. preliminary. sweep indicated -71-

that the peaks were all below 0.35 Mev. The elctron spectrum below this energy is shown in Figure 26, l.m.ich demonstrates the complexity of the mixture. Realizing that there was a considerable quantity of 18 hour ,- Pb200 present from the reaction T1203(d,5n)Pb200, the decay of the promi- •,. nent electron lines was followed. ,J. run about three days later is shown in Figure 27, in which several lines have definitely decayed with a

shorter half-life. Qn this basis, Kl, Ll, and Ml are assig~ed tcr a 0.139 Mev gamma ray of Pb200, while K2, ~, and N2 are the internal

conversion electrons of the 0.269 Mev gamma ray of p~03. The lines K3 and L3 decay at, about the same rate as Ll, and correspond to internal conversion lines 6f a 0.320 Mev gamma ray of Pb200 • The line K4 is longer lived than 1J and seems to decay with about the same half-life as the L line of the 0 .. 269 Mev 'gamma ray of Pb203 • Since all other lines in this energy region can be accounted for, K4 is probably the K line of a 0.424 Mev gamma ray. Further, as the lines due to the soft

0 0 139 Mev gamma ray of p~OO decrease in intensity, a new line, K5 is

noted. It is assumed that this is the K line of a third gamma r~ of Pb203, having an energy of 0.153 Mev. Thus one is led to postulate a decay scheme in which the electron

capture in p~03 excites a level about 0.422 Mev above the ground state of T1203, and that this state emits either a 0.422 Mev 'gamma ray, or a

,>, 0.269 Mev and a 0.153 Mev gamma ray in cascade. The measurement of Lutz, Pool, and Kurbatov62 is therefore con- firmed for the 0.269 Mev gamma ray, and it appears that the beta decay of Hg203 and the electron capture in Pb203 indeed excite different energy .' levels. Also, the decay scheme of Pb203 is considerably more complicated -72-

! '

",. 10

o~-L--~~---L--L-~~~~--~--~-- 0.3 0.6 MAGNET CURRENT, AMP Mut752

Fig. 26, Electron spectrum of Pb200,203 mixture -73-

203 K2 Pb l 7 .., ,., 269 KEV 6 422 KEV---'-----'-_153 KEV T,203 ' 5 N 4

O~~~--~-L~L-~~~~~~C=~~~ 0.4 0.6 0.8 1.0 1.4 MAGNET CURRENT,AMP.

MUI753

Fig. 27 Pb200,203 electron spectrum, observed three days after " Fig. 26. Also shown is the decay scheme proposed .' for Pb203 • -74-

than the Hg203 . decay. Further evidence that the 0.269 Mev gamma ray and the 0.279 Mev gamma ray represent different transitions is seen from the ratio of K electrons to L electrons. SlRtis and Siegbahn61 ,- report NKlNL = J for the ~g203 gamma. ray, while the 0.269 Mev gamma •,. ray of Pb203has a NKlNL = 2.3 after correcting for absorption in the

counter window o It is possible that this variation represents a significant difference in the character of the radiations. No internal conversion electrons were seen which could be inter preted as lines of a 0.47 Mev gamma ray from the decay of Pb203 • However, Lutz, Pool, and Kurbatov62 obtained this gamma ray energy from the endpoint of a Compton electron distribution, uncorrected for spectrometer resolution. This procedure is known to give high results63 and it is pOssible that this gamma ray is actually the 0.424 Mev gamma ray observed in these experiments. The decay scheme proposed for Pb203 is included in Figure 27.

G. Beta Energy of Pb209

The diSintegration energy of p~09 is especially interesting because of the apparent descrepancy arising from a cycle involving the neutron .binding energies of Pb207,Pb208, Pb209, Bi210, and the decay energies of Pb209 , Bi210, and Po210. Consider the follOwing closed cycle:

Pbf06 + Mn "" Pb207 + 6.70 Mev Pb207 + Mn ~ Pb208 + 7.37 . p~08 +.Mn "" Pb209+ 3.88 Pb209 "" Bi209 + E~l Bi209 + Mn "" Bi210 + 4.18 Bi210 "" Po210 + E132 p0210 =pb206 + Ma + 5.;30 Mev -75-

Adding the above equations, the heavy masses canoel, and there remains

4Mn - Ma:. = 27.43 + El31 + Ei32 Using values for EPI and Ef32 of 0.66 and 1.17 Mev respectively, the " right member becomes 29 .;36 Mev, compared with 29 .81 Mev for the left .•, member. The neutron binding energies used were those of Harvey,64

while the Bi2lO (RaE) and Po2l0 energies have been widely investigated and may be considered well known. Thus a .discrepancy of about 004 Mev

results if the disintegration energy of Pb209 is taken as 0.66 Mev. Harvey64 believes this discrepancy is due to a gamma ray of about 0.4 Mev which follows the beta transition. The purpose of the experiment to be described was to look for internal conversion electrons corresponding to gamma rays of about this energy, and to remeasure the beta energy.

The sample of p~09 activity was prepared b.1 irradiation of ordin-

ary lead with 18 Mev deuterons from the Crocker Radiation Laboratory cyclotron anc was chemically purified by H. M. Neumann. Since the sample was not carrier free, the beta spectrometer source was quite thick (several mg/cm2), and low energy lines could not have been seen. No electron lines were detectable up to 1.0 Mev. Neumann found no evidence for a gamma ray of about 004 Mev, using absorption techniqUes;65 the aluminum absorption curve of the beta particles dropped a factor

of 1200 to bremsstrahlung, indicating that any gamma rays present were of low abundance. However, any gaIinna ray postulated to be present in

low abundance would not be in cascade with the main beta group, and hence would not resolve the question of the supposedly lO1ildecay energy of Pb209 • -76-

A Fermi-Kurie plot of the beta spectrum is shoW? in Figure 28, and

is apparently complex, with beta components of 0.66 ± 0 001 Mev and , r

0.53 ± 0 0 01 Mev. It is obvious from the Fermi-Kurie plot that the source was thick, at least in spots, because of the sudden absorp- •" tion effect at about 300 Kev, causing the curve to drop toward the energy axis. Because of the thick source, the K line from a possible

0.13 Mev ,gamma r~ could not have been seen, and the L line might have been in such low abundance that it was missed. It is therefore reasonable to find a two component beta spectrum without seeing the

associated gamma r~.However, it must be stated that no effort was made to correct for the thick source after the manner of Owen and Primakoff,17 since one would expect the Fermi-Kuri plot to be affected at energies considerably below 0.5 Mev.

t. Thus, it is difficult to account for the discrepancy in the closed

cycle by a higher p~09 decay energy. The most probable source of error would seem to be the measurements of the neutron binding energies.

HarveyVs value of 3~88 Mev for the binding of a neutron,between p~OB_ Pb209 constitutes the only measurement of this quantityo The other

energies used in the closed cycle calculation have been found by at least two investigators and are believed to be more acourate. However" Kinsey66 reports that the gamma ray peak from the (n,r) reaction on 209 't Bi is very broad, which m~ indicate that his measurement as well as Harveytsmeasurement might not involve the ground state transition

and would hence be low 0

I', -77-

0.66 MEV 1 300 400 500 600 700 KEV

MIl 1754

• . Fig. 28 Fermi-Kur~e plot of Pb209 beta spectrum -78-

H. Neutron Deficient Isotopes of Zirconium arid their Decay Products This section concerns work done with Dro E. K. Hyde, who. has estab lished radiochemically the decay schemes discussed below. The present author's work on the energies of the various decay processes will be treated here. A more detailed account of the radiochemical and spectroscopic research'wi11 be pub1ished.67

It is possible to prepare a number of the lighter neutron deficient isotopes of zirconium by bombarding high purity niobium metal wi th 100 Mev protons in the 184-inch cyclotrono The lightest molybde num and niobium isotopes produced by (p,xn) and (p,pxn) reactions decay quickly by electron capture or positron emission to isotopes of

zirconium 0 Also, zirconium isotopes maybe produced by spallati9n reactio?s of the type Nb93(p,2pxn)Zr<93. Such reactions allow pre ". paration of isotopes of zirconium of mass number 89 or less, free from higher mass zirconium activities.

Zirconium 87 .-The decay s.cheme which best fits the information for Zr87 is presented in Figure 29. The decay curve of a zirconium

fraction from a proton irradiated niobium target is complexo A prominent activity with a half-life of about 90 minutes is observed, although reliable resolution of the curve is difficult because the

growth and decay of yttri~ and strontium daughter activities is super imposed upon the straight decay of 78 hour Zr 89, 17 hour Zr86 , and the short lived activity which is due to Zr87. The nature and energies of the radiations did not allow simplification of the resolution byfol-

lowing the decay through properly selected absorberso The half-life -79-

T/ -94±6min 12 + . f3 2.IO± 0.02 Mev . 87m ~-y .

TI/2 .. 14 hrs Complex ., Spectrum IT in Region> i.0 Mev 0.389 ± 0.004 Mev., ..-I1.io- y 87

T~2 -80 hrs EI ect ron Capture Sr 87 m

T1/ " 2.80 ±O. 03 hrs 2 I''T

0.394 ± 0.004 Mev "...... II...... Sr 87 Stable

MU ,184

Fi'g. 29

Decay scheme of Zr87 -80- of Zr87 was measured by placing a sample in the beta spectrometer and following the decay of positrons of an energy greater than the 0.910 Me; positrons of Zr89. A typical decay curve is shown in Figure 300 The points have been corrected for counter background. As a result of several determinations a value of 94 ± 6 minutes is obtained for the half-life of Zr87. This value is considerably shorter than the 120 ± 6 minute half-life found by Robertson ~ al.68 In preparation .for a measurement of the Zr87 positron energy, the beta spectrometer was calibrated wi th the K internal conversion line of the Cs137 gamma. ray, and the calibration was checked by determining the energy of the p32 beta particleo A value of

1~685 ± 0001 Mev was obtained, in excellent agreement with the recent value of Langer and Price.69 The resolution of the spectrometer was 1.5 percent. The Fermi-Kurie plot of the Zr87_Zr89 mixture was resolved .into

the 2 0 10 ± 0 0 02 Mev component of Zr87 and the 0 0 910 ± 0.010 Mev com ponent of Zr89 and no others. The portion of the plot beyond the endpoint of zr 89 is shown in !igure 31. The value for Zr87' agrees with that of Robertson et aL68 within the experimental error of

their absorption measurements 0

An examination of the electron spectrum showed no prominent con 87 version electrons which could be assigned to Zr 0 In addition, Hyde found no evidence for Zr87 gamma radiation, using lead absorption techniques on freshly purified zirconium fractions; however, it was not possible to set very low limits by either method. Robertson et al.68 found gamma rays of 0.35 and 0 .. 65 Mev energy, and x-radiation ,,:,,81-

~, 4000

~~ • \ I \ 1000 \ \

LIJ I- ::> -94 MINUTES z \-li• 1/2 :IE ~ a:: \\ a..LIJ 1\\

fI) l- \\x \ . z ::;, 100 '. 0 \~~. (.) >- \~ l-- x • ..... " > xx l- x ~ (.) \ •

10 '\ 0 4 8 TIME MUIIB6

Fig. 30 Half-life of Zr87 determined by measuring decay of monoenergetic positrons of le47 Mev in the beta spectrometer. -82-

( ,

O~----~------L--~------2.4 3.0 4.0 5.0 .... ", 2J l, rna c MU 1185

" Fig. 31 Fermi-Kurie plot of high energy portion of Zr87 positron spectrum -83- accompanying Zr87 decay ••

, Decay Products of Zr87 .--If an yttrium fraction is separated from the purified zirconium within two or three hours after bombardment, the yttrium fraction_ is almost 100 percenty87m with a nearly unde 67 tectable trace of y86 and y88. Radiochemical evidence. ,indicates that the y87m undergoes isomeric . transitions, : forming y87.. The ground state y87· decays by pure . electron capture to Sr87m, which in turn· emits· a single gamma ray to form stable Sr87"

Carrier free samples of y87m freshiy isolated from the Zr87 parent were mounted. on tllin Tygon films and examined in the beta spectrometer. The most prominent radiations were the conversion lines (Figure 32) of the 0.389 Mev gamma ray" The Kit ratio was

8.3 ± 0.5. The half-life and K/L ratio suggest electric 25 pole radiation.

Mann and Axe170 have measured the K conversion line of this gamma ray to be 0.374 Mev, which would indicate a gamma ray energy of

0 • .391 Mev 0 However, they state that in only. 50 percent of its disintegrations does y87m decay by isomeric tranSition, while in half the disin~egrations y87m decays directly to Sr87m. They place the 0.39 Mev gamma ray in this transition. It is difficult to fit the radiochemical .evidence of Hyde and 0'Kel1ey67 to this scheme.

Robertson ~ al.68 report that 14 hour y87m decays by positron emis sion and not by isomeric transition and state that y87m and y87 decay independently toSr87m• From the evidence below, this cannot

,.. :- -84-

,.,i EK = 371.5 KEV .~ 20 !" = 388.6 KEV t,

It) I 10 .. 0 X

t. Q.- (/) :E I- - 0 z- EL =387 <.,;)

:E '!' E., = 389

o I. 30 I. 32 1.34 1.36 1.38 1.40 MAGNET CURRENT, AMP MU 1126

Fig. 32 Conversion electrons of the 0.389 Mev gamma ray of y 87rJl

'.f .', -85-

be the case o It is possible that Robertson and co-workers were measuring the 1406 hour yttrium activity which is assigned to y86 (see below), which ill their bombardments would have appeared through the Sr84(a~pn)y86 reactiono I~ Figure 33 shows the decay of the conversion electrons of the y87m gamma ray, and .the growth of the conversion electrons of the Sr87m granddaughter~.·o Figures 32 and 33 represent the same sample examined approximately 3 hours, .28 hours, and 45 hours after pre- 87m paration of the pure y87m sample 0 • The Sr gaJllIIla ray energy is

00394 ± 00004 Mev; and is definitely higher in energy than the gamma ray of y87m (Figure ,32) • Mann and Axel70 report the Sr87m gamma ray 87m energy to be 0 0 390 Me:vo . The K/L ratio of the Sr gamma ray is 702 which is in agreement with Mann and Axel's70 value of 6 .. 9, within

.~ experimental erroro Wi thin experimental error, the area under the K line of y87m I. decayed with the proper 14 hour half-lifeo The sum .of the areas

under all four peaks of Figure 33 decayed ina manner similar to the curve expected for a 14 hour decay plus the growth and decay of a 2 80 hour granddaughter having a counting efficiency (conversion 0 \ coefficient) about 20 percent lower than the 14 hour activityo In this calculation it was assumed that the daughter of the 14 hour activity was counted with zero countingefficiencyo This indicates

that the Sr87m is formed completely through the 00389 Mev transition

in y87m, and that the decay scheme of Figure 29 is correct 0 Similar results are obtained when an ordinary Geiger counter is used to follow the decay, since th'e conversion electrons of tr'm and Sr87m are counted -86-

87m Y oJ K 8

87m Sr 6 ~ EK=377.5 KEV E" = 393.6 KEV

II) '0 4 2 87m )( Sr ,~ K (/) I- Z Z ::::l 0 :E 2 (.)

o ~--~~~~~~~~--~~~~ 1.30 1.34 1.38 1.30 1;34 1.38

MAGNET CURRENT, AMP MU 1125

Fig. 33

The conversion electron s~ectrum 28 hours and 46 hours after preparation of the pure y87m sample, showing growth of con version electrons of 0.394 Mev gamma ray of Sr87m -87-

efficiently, while the x-rays from the electron capture in y87 are not detected appreciably.

In addition to these prbminent lines, an examination of the electron sp~ctrum of freshly prepared y87m showed eight-internal conversion lines in the region above 1.Mev. Tentatively, these are

assigned to gamma rays of 1.07, 1:15, 1044, 1049, 1.54, 1060, 1 0 66; and 1.71 Mev energy. No detailed study has been made of these electrons but they appear to decay with a 14 hour half-life. The total num-

b~r of positrons ~s <0 0 001 times the number of conversion electrons. No conversion electrons were observed for the 80 hour y87 other than Auger E3lectrons. Lead -absorption measurements on y87m show considerable hard gamma radiation of energy greater than 1 Mev, but no hard radiation is found for y87 or Sr87ID• Mann and Axel report that the

.~ K capture decay is followed by the emission of a 0 .. 485 Mev· gamma ray. We failed to see these conversion electrons in our spectrometer

studies, but the conversion'coefficient reported by these authors is so small that they could have been overlooked in these experiments.

Zirconium $9.--If a purified zirconium fraction, from proton'

bombarded niobium is allowed to decay for several d~s, it is found that 78 hour Zr89 is the predominant activity remaining after repuri-

\ fication. -By this time the other zirconium activities have decayed out with the exception of Zr88, which because of its long half-life and inefficiently counted x-radiation contributes only a few percent

to the tot,al radiation ~t this time. -88-

Qarrier free Zr89 samples were examined in the spectrometer.. A

Fermi-Kurie plot of the positron spectrum gave a value of 0~910 ± 00010 Mev .. This value is lower than the value of 1007 Mev obtained by Overstreet, . Jacobson, and Hamilton by aluminum absorption.

'. Four groups of conversion electrons were observed, corresponding r '. to gamma ray energies of 0 0 027 ± 0~001, 0 0 396 ± 0 .. ,004, 0 0917 ± 00005,

and 1027 ± 0~01 Mev. The electron spectra of the iast three gamma rays are sho,m in Figure 34. It is apparent that the decay scheme of Zr89 is complex, although it seems probable that a single positron ,transition occurs to a level about 1031 Mev above the ground state of y89, and that this excited state decays either qy the emission of

00396 and 0.,917 Mev gamma rays in cascade, or qy 1.27 and 0 0 027 Mev gamma rays in cascade.

yttrium 86 .. --If a purified zirconium fraction is allowed to decay for about a day and then repurified,no more of the mass 87 chain daughters are formed.. Subsequent removal of an yttrium fraction after the remaining zirconium had decayed for another day yields con siderable y86 positron activity, which decays Yith a half-life of

14.6 ± 0 .. 2 hours~67 Less than 001 percent of the yttrium activity is due to y88 0 The nuclide y86 was discovered by Castner72 in bombardments of strontium isotopes. ,Lead absorption experiments indicate a hard gamma ray of 104 Mevo 67

• A Fermi-Kurie plot of the positron spectrum of y86 is shown in Figure 350 The data are corrected for instrumental distrotion according to Owen and Primakoff.17 Two components are present in about equal -89-

900 KEV ~ K2 E~2= 917KEV 379 KEV 5000 1.256 MEV K, K3 , E~: 396 KEV E~3' 1.27 MEV I !. 900. ISO 4000

3000 ,\. a. 600 120 ~

1000

______.L-L...JL..--.I---.1.~--'--ll!l~ ______L-L...J--L---.1.--L~..J 1.42 2.36 2.40 2.44 2.4S 3.0S 3.123.16 3.20 , MAGNET CURRENT, AM~

Fig. 34 Conversion electrons of Zr89 -90-

2 Eo·3.33 mo c Eo· 1.19 ± .02" MEV G 2 Eo' 4.52 mo c ...... -.-.. Eo -1.80 :t .02 ME;;I. 3.5 4.0 4.5 5.0

MU 1189

Fig. 35 Fermi-Kurie plot or y86 positron spectrum -91-

intensity, with .maximum energies of 1 0 80 ±0002 and 1019 ± 0002 Mevo There are not sufficient data on the high energy component to establish its shape, but the low energy group appears to be forbidden. The comparative half-life of the low energy positron (ft~106) indi- ,.~ cates a first forbidden transition, while the spectral shape suggests ...... a spin change of two units and a change of parity. According to the theory of forbidden spectra,73,74 dividing the ordinates of the plot

of the low energy component of Figure 35 by a1/ 2 should yield a straight

line if the transition is of this type 0 The factor a 1:0 (e?- - 1) + (co _c)2,

wi th c the energy in units of moc2, arises from the unique energy dependence of the transitione Figure 35 shows such a corrected plot, and ft:i..s seen that a straight line.does result. 2 The factor c '= (ez - 1)2 + 10/3(c - l)(co _c)2 + (co _c)4 gave '. a poor fit. Conformity to this factor wuldhave indicated a second forbidden transition with a spin change of three uni tsand no change in parity. Internal conversion conversion lines were sought, but none were seen. However, the samples were of comparatively low intensity, and the presence of gamma radiation oth.er than a hard 104 Mev gamma ray , . seen in lead absorption experiments cannot be ruled outo -92-

IV. ACKNOl.JLEGMENT The work described in this dissertation was performed under the direction of Professor Go T. Seaborg, whose thoughtful advice and encouragement are gratefully acknowleged. The assistance and advice of Mr. F. 10 Reynolds in the construc- . tion and opere.tion of the beta spectrometer is greatly appreciated.

I wish to thank Mr. Ao Garren for the design of the beta spectrometer magnet polepieces, Mr. H. Po Hernandez for the engineering design of the magnet assembly, Messrs. H. D. Farnsworth and E. Powers for supplying the necessary electronic equipment. I am indebted to Professor 10 Perlman for many suggestions and discussions, and to Dr. E. Ko Hyde, Mr. W. W. T. Crane, Dr. Do Ao Orth,

Dr. D. Go Karraker, Dr. Ho Mo Neumann, Dr. Go 11. Barton, ~d

Mr. c. I. Browne, Jr .. , for assistance in many of the experiments 0 This work was performed under the auspices of the U. S. Atomic • Energy Commission. -93- V. REFERENCES

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